Peroxidically crosslinked silicone elastomers, generally referred to as HTV (high temperature vulcanizing) silicone rubber have a certain, undesirable yellow coloration absent from addition-crosslinked silicone elastomers. This yellowing is very well-known, and numerous attempts have been made to prevent or reduce this undesirable coloration. It is known that yellowing depends on numerous influencing factors which explains why the degree of yellowing varies greatly. For example, peroxidically crosslinked silicone elastomers are only slightly yellowish immediately after preparation, i.e. in the untempered state. However, after thermal post-treatment, for example for 4 hours at 200.degree. C., which is indispensable for many applications, yellowing is particularly pronounced. It is also known that the type of peroxide used has, for an otherwise unchanged composition, a considerable influence on the degree of yellowing. Thus, silicone elastomers which have been crosslinked by means of dicumyl peroxide are significantly more yellow than those which have been crosslinked using bis(2,4-dichlorobenzoyl) peroxide.
JP 04161458 (S ETSU CHEM IND CO LTD) describes the use of specific peroxides, namely dialkyl peroxides, in combination with an inorganic or organic alkali metal base such as sodium hydroxide, sodium silanolate or sodium methoxide, with the latter being reacted with the filler-containing organopolysiloxane composition at elevated temperature before the addition of peroxide. A disadvantage of this process is, inter alia, that it is generally restricted to certain peroxides because of the different methods of preparing the silicone elastomers (extrusion or compression molding). In other words, there is no free choice of peroxide. In addition, the basic alkali metal compounds added leads to a series of undesirable secondary reactions, results in disadvantages in the process, can increase the tendency of the compound to stick to the rolls, can cause difficulties in removal of the crosslinked silicone elastomer from the mold, and can have an adverse effect on the final properties of the silicone elastomers, e.g. compression set and thermal stability.
A further factor which causes and/or influences yellowing is the presence of nitrogen-containig residues in the HTV polymer or filler used. These residues can originate from the PNCl.sub.2 catalyst used in the preparation of the polymer or from silazane residues or products formed from them. Factually, yellowing can in many cases be reduced a limited amount by intensive kneading at elevated temperatures i.e. careful elimination of such residues, but this is associated with additional process steps which are not economically acceptable.
It would be desirable to provide additives which can be added to the uncrosslinked HTV silicone rubber composition and reduce yellowing. According to the prior art, the additives disclosed by the following references are known to be useful for this purpose.
DE 3726010 (WACKER-CHEMIE GMBH) describes peroxidically crosslinkable silicone rubber compositions to which organopolysilanes are added to reduce yellowing. Although a significant reduction in yellowing can be achieved in this way, considerable disadvantages have to be accepted; these relate, in particular, to significantly higher compression set values and an increase in the surface tack and adhesion, which can be associated with processing disadvantages (tendency to stick to the rolls, poorer demoldability) and undesirable final properties (surface tack).
DE 3243194 (DEGUSSA AG) teaches the use of polymethylhydrogensiloxane (H-siloxane) which is added to the peroxidically crosslinkable silicone rubber composition in amounts of from 0.1 to 2% by weight (based on the total composition) as an additive to reduce yellowing. In numerous cases, yellowing can be significantly reduced or even eliminated completely in this way. However, it has been noted that frequently, significantly higher amounts of H-siloxane than the amounts disclosed as useful must be used to achieve a satisfactory reduction in yellowing. In these cases in particular, but also in other cases, the use of the relatively low molecular weight polymethylhydrogensiloxanes creates disadvantages in processing which result, first in an increased tendency of the compoumd to stick to the rolls, and second, in difficulty in removing the crosslinked silicone elastomers from the mold (also referred to as mold adhesion). This is understandable since H-siloxane is known to be used as a coupling agent.
It is known that the tendency to yellowing can be reduced by intensively kneading the silicone rubber composition at high temperatures (&gt;150.degree. C.) for a number of hours. The occurrence of yellowing is then a particular disadvantage if the process for preparing the silicone rubber composition does not include such a (time-consuming and energy-intensive) baking-out step. In these cases in particular, relatively high amounts of H-siloxane are necessary to reduce yellowing and the associated disadvantages become particularly serious.